In producing photographic film or paper, it is necessary to coat the film or paper support with discrete layers of photographic coatings. Some of these layers contain a radiation sensitive material like silver halides, diazonium salts, and light sensitive dyes as well as other photographic additives including matting agents, developing agents, mordants, etc. Other layers may contain materials which are not radiation sensitive like subbing layers, pelloid protective layers, filter layers, antihalation layers, and interlayers. Additionally, hydrophilic colloids, polysaccharides, surfactants, and synthetic polymers may also be incorporated in photographic coating liquids.
The number of separate and discrete layers of photographic coatings applied to photographic paper or film support depends on the product's design. Typically, the number of layers varies between 1 to 15, more usually 3 to 13.
A slide hopper is a known apparatus which will coat one or more liquids onto a solid support. The conventional multi-slide hopper performs its coating operation by metering a first coating liquid from a supply through a narrow slot which distributes the liquid uniformly across the top of a downwardly inclined slide surface. This layer of liquid moves down the slide surface by gravity to supply a steady, uniform, smooth coating layer to a coating bead across which it is applied to a moving web being coated. A second coating liquid is supplied to and distributed by, a second slot which directs a uniform layer of that liquid onto the top of a second slide surface. The second coating liquid first flows down its own slide surface and then onto the top of the layer of liquid issuing from the first slot without interlayer mixing. The layers of the first and the second liquids then together flow down to a coating bead where they are applied to the web. Additional liquids may be coated simultaneously by equipping the hopper with the appropriate number of slots and slide surfaces.
Instead of applying photographic coatings from a multi-slide hopper to a web by use of a coating bead, multi-layer photographic coatings can be applied by passing the web beneath a liquid curtain formed by discharging the coating liquid from a terminal lip portion of the multi-slide hopper. Both the bead coating and curtain coating techniques are well known, as disclosed e.g., in U.S. Pat. No. 4,287,240 to O'Connor.
In older photographic coating hopper arrangements, photographic liquids were pumped from a narrow feed conduit into a distribution channel where the liquid was spread transversely across the hopper. From the distribution channel, the photographic liquid was passed through a metering slot of constant width and discharged onto a slide surface. U.S. Pat. No. 2,761,417 to Russell et al. depicts such a system.
It has been found that such conventional hoppers often tend to produce a defect in the final coating product which appears as a long line or lines running parallel to the direction of coating. These defects are not always visible in the product as coated and very often they become visible only after the product is dried and/or processed (if the coating web is a photographic product) and then is visually checked. One cause of such streaks is local deficiencies in the layer of coating liquid issuing from any slot which is thinner than the adjacent layer of coating liquid. The total thickness of the layers is constant throughout the coatings. Streaks may also result from the entrapment of particles and bubbles in areas of the coating system having low wall shear stress or regions of recirculation (i.e., vortices).
In U.S. Pat. No. 3,005,440 to Padday, the line problem was attacked by terminating the metering slot at a discharge slot which abruptly widens at a right angle to the metering slot. This sharp right-angle corner produces the maximum amount of turbulence in the stream and heals lines formed by upstream blockages in the metering slot. With this configuration, any flow obstructing particles will be present only in the metering slot having a narrow width to maintain backpressure on the upstream distribution channel. The length of the wider discharge slot is sufficient to heal any turbulence created by blockages in the metering slot. Studies, however, indicate that streak-creating vortices can occur in the discharge slot at slot Reynolds Numbers of 5 or above.
Another approach to elimination of lines, as discussed in U.S. Pat. No. 3,474,758, is to direct the exit end of the discharge slot at an angle to the slide surface on which liquid from the slot exits. Somewhat similar to this concept is the device disclosed in U.S. Pat. No. 4,041,897 to Ade where each emulsion is applied to a slide on the device through a slot having a vertically-extending upstream wall and an inclined downstream wall such that the slot widens as it approaches the slide. Such techniques, however, are susceptible to the adherence of streak-forming particles to the incline.
Streaking is thus a significant problem in processes of coating a pack of photographic emulsion layers onto a support. There continues to be a need for an economical and effective procedure for correcting this problem.